Nonlinear microrheology of wormlike micelle solutions using ferromagnetic nanowire probes

Ferromagnetic nanowires were employed to investigate the microrheology of wormlike micelle solutions composed of equimolar cetylpyridinium chloride–sodium salicylate. For a wire rotated about a short axis, the drag at low rotation rate ω_R is independent of ω_R and strongly temperature dependent, consistent with the macroscopic shear viscosity. Above a critical rotation rate ω_c, the drag is independent of temperature and decreases as a power law with increasing rate. The onset of nonlinear drag is characterized by a peak associated with contributions from extensional flow. Above ω_c, the fluid generates an additional torque that tilts the wire out of regions of high shear flow and that is interpreted as a consequence of a shear-induced transition to nematic order among the micelles. Rotation of the wire in response to this torque reveals directly the anisotropy of the drag in the nonlinear state.